Semen production data of Holstein-Friesian and Jersey bulls collected over a period of three years was analyzed to examine seasonal effects on quantity and quality of semen. Purebred breeding bulls of Holstein-Friesian and Jersey breeds (n = 18 for each breed), maintained under naturally ventilated open-sided sheds, were used for semen collection. Meteorological information on ambient temperature, relative humidity and rainfall was used to divide the calendar year into two stressful summer seasons viz; dry summer (April - June) and wet summer (July–September) and a stress free season (October - March). The number of ejaculates per bull recorded during three seasons were not significantly different (P>0.05) in both breeds. Friesian bulls produced lower ejaculatory volume during dry summer season, whereas Jersey bulls produced higher (P<0.05) volume during wet summer compared to other seasons. Seasonal pattern of mass motility and individual motility of semen was different between two breeds. The mass motility of semen in both breeds was significantly lower during wet summer. However, individual motility in the semen of Friesian bulls did not differ among seasons (P>0.05) but in Jersey bulls it was lower during wet summer than other seasons. Wet summer resulted in reduced number of semen doses frozen per bull in both breeds. The results of this study indicated that wet summer season deteriorated semen quality in terms of mass motility, individual motility and number of doses in both breeds, except individual motility in semen of Friesian bulls.

Key words: Bulls, Holstein-Friesian, Jersey, Semen quality, Tropical environment